A. Field of the Invention
This invention relates to the subject of mobile Internet Protocol (“IP”) data networking.
B. Description of Related Art
Public packet switched networks can be used to carry traffic to and from a mobile communications device, such as a laptop computer or personal digital assistant equipped with a cellular telephone modem. The basic architecture of mobile IP data networking is known in the art and described in several publications, including the Request for Comments document RFC 2002 (1996) and in the textbook of Charles E. Perkins, Mobile IP Design Principles and Practices, Addison-Wesley Wireless Communications Series (1998), the contents of both of which are incorporated by reference herein.
Basically, in Mobile IP communication, a wireless mobile node communicates with a terminal on an IP network by means of a foreign agent and a home agent. Typically, foreign agent functionality is incorporated into a network access server chassis located on a mobile node's visited network. The foreign agent provides routing services for the mobile node while it is registered with the foreign agent. A Point-to-Point Protocol (PPP) session is established between the mobile node and the foreign agent. The foreign agent de-tunnels and delivers datagrams to the mobile node that were tunneled by the mobile node's home agent.
The home agent is usually a router on a mobile node's home network that tunnels datagrams for delivery to the mobile node via the foreign agent when the mobile node is away from home. The home agent maintains current location information for the mobile node, through a variety of possible mechanisms, such as described in the patent application of Richard J. Dynarski, et al., “Dynamic Allocation of Wireless Mobile Nodes Over an Internet Protocol (IP) Network”, Ser. No. 09/233,381, which is incorporated by reference herein. When multiple home agents are handling calls for multiple mobile nodes simultaneously, the home agents are providing, in essence, a service analogous to virtual private network services. Each mobile node is typically associated with a separate home network. The routing path from that home network, through the home agent, to the foreign-agent and mobile node is like a virtual private network for the mobile node.
The situation can arise in which two mobile nodes are simultaneously registered with a foreign agent, and have established a PPP session with the foreign agent, but the two mobile nodes have the same home network IP address. In a typical prior art foreign agent implementation, the overlapping IP addresses will cause an IP routing failure, since more than one routing table entry could have the same network address associated with different PPP links. This overlapping IP address for both mobile nodes could be a consequence of the home network assigning the same IP address to the mobile node, either a priori or during the mobile IP registration process.
Accordingly, there is a need in the art for a method for handling overlapping IP address pools in a foreign agent. The present invention solves that need and provides methods by which a foreign agent can correctly route packets to a from any number of mobile nodes that are simultaneously registered with the foreign agent but have the same home network IP address.